An image used to define areas of transparency in another image. See also compositing, effects.
master
The source image file copied from a computer's file system or a camera's capture card. In Aperture, the master file is never altered. Anytime a change is made to the image, that change is applied to a version. See also project, version.
master Library
The place where master images are kept. The default location is in the Pictures folder of the home directory, but the Library can also be on an external hard disk. See also vault.
matte
An effect that uses information in one layer of a digital image to alter another layer. Matte filters can be used by themselves to mask out areas of an image, or to create alpha channel information for an image to make a transparent border around the image that can be composited against other layers. See also compositing, effects.
megapixel
One million pixels. For example, 1,500,000 pixels is 1.5 megapixels. See also pixel.
metadata
Data describing other data, including image files. Databases use metadata to track specific forms of data. Aperture automatically extracts all industry-standard EXIF and ITPC metadata when importing images and also lets metadata such as copyright, captions, and keywords to be added. See also EXIF, IPTC.
metadata inspector
Where all the information associated with an image is displayed in the Aperture interface. Industry-standard metadata can also be added in the inspector.
midtones
The values in an image between absolute white and absolute black.
mirroring
Displaying the same image on two or more monitors. See also extended desktop mode.